To increase reliance on renewable energy supplies such as solar and wind power, it is necessary to increase the amount of energy storage systems connected to the electrical grid. Of the electrochemical energy storage (EES) systems under consideration for stationary storage, redox flow batteries (RFBs) are of immense interest (Weber et al., J. Appl. Electrochem., 2011, 41, 1137-1164; Leung et al., RSC Advances, 2012, 2, 10125-10156; Parasuraman et al., Electrochim. Acta, 2013, 101, 27-40; Shin et al., RSC Advances, 2013, 3, 9095-9116; Wang et al., Adv. Fun. Mater., 2013, 23, 970-986; Alotto et al., Renewable and Sustainable Energy Reviews, 2014, 29, 325-335). Aqueous-based RFBs containing vanadium complexes have been commercialized on scales as large as 5 MW (“Rongke Power 5 MW/10 MWh VFB Energy Storage System successfully finish power transmission to Liaoning Power Grid,” www.rongkepower.com/index.php?s=/article/show/id/140/language/en, Accessed Jul. 20, 2015). Through the replacement of aqueous components, which limit charging potentials to 1.5 V due to the working electrochemical window of water, with organic materials, it may be possible to develop batteries with charging voltages as high as 5 V.
Despite decades of research on the use of organometallic compounds as electro-active materials in non-aqueous RFBs, most systems have been limited by low solubility, poor capacity retention, and/or low faradaic efficiency (Soloveichik, Chem. Rev., 2015, ASAP article). Few examples of highly soluble species have been reported, and even in these cases, testing has been limited to concentrations too low for practical use in commercial applications (Cappillino et al., Adv. Energy Mater., 2014, 4; Cabrera et al., J. Phys. Chem. C, 2015; Suttil et al., J. Mater. Chem. A, 2015, 3, 7929-7938; Hwang et al., Chem Sus Chem, 2015, 8, 310-314). More recently, reports of non-aqueous RFBs containing organic electro-active materials have surfaced. N-oxidanyl amines (e.g. TEMPO), dialkoxybenzenes, and phenothiazines serve as electron-donating electro-active materials, while phthalimide, anthroquinones, quinoxilanes, fluorenone, and viologen act as electron-accepting counterparts (Li et al., Electrochem. Solid-State Lett., 2011, 14, A171-A173; Wei et al., Adv. Mater., 2014, 26, 7649-7653; Wei et al., Angew. Chem. Int. Ed., 2015, 54, 8684-8687; Brushett et al., Adv. Energy Mater., 2012, 2, 1390-1396; Kaur et al., Energy Tech., 2015, 3, 476-480; Wang et al., Chem. Commun., 2012, 48, 6669-6671; Nagarjuna et al., J. Am. Chem. Soc., 2014, 136, 16309-16316). Many of the electron donors have been used as electron-transfer catalysts in other energy storage and collection applications, including redox shuttles for overcharge protection of lithium-ion batteries (LIBs), electron-transfer agents in lithium-air batteries,27,28 and redox mediators in dye-sensitized solar cells, among others (Chen et al., Electrochim. Acta, 2009, 54, 5605-5613; Balakrishnan et al., J. Power Sources, 2006, 155, 401-414; Buhrmester et al., J. Electrochem. Soc., 2006, 153, A288-A294; Ergun et al., J. Phys. Chem. C, 2014, 118, 14824-14832; Ergun et al., Chem. Commun., 2014, 50, 5339-5341; Kaur et al., J. Mater. Chem. A, 2014, 2, 18190-18193; Narayana et al., Chem Phys Chem, 2015, 16, 1179-1189; Chen et al., Nat. Chem., 2013, 5, 489-494; Lacey et al., Electrochem. Commun., 2013, 26, 74-76; Hamann et al., Energy Environ. Sci., 2011, 4, 370-381).
N-ethylphenothiazine (EPT, FIG. 1) is a particularly stable electron-donating compound that oxidizes at ca. 3.5 V vs. Li+/0 in carbonate-based electrolytes. This commercially available material survives extensive overcharge cycling in LIBs. Studies of EPT show that it is stable in aprotic, organic solvents in the neutral and the singly oxidized (radical cation) states (Odom et al., Energy Environ. Sci., 2014, 7, 760-767). It was considered whether these characteristics could allow EPT to serve as an effective electro-active material in non-aqueous RFBs as a one-electron donor. However, EPT's limited solubility in organic solvents (ca. 0.1 M) makes it impractical for this application, which requires electro-active material concentrations of 1 to 2 M to be competitive with the capacities of aqueous RFBs. In comparison, it was reported that the EPT derivative 3,7-bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) dissolves at concentrations as high as 1.5 to 2 M in organic solvents and electrolytes and is even more stable than EPT (Odom et al., MRS Online Proceedings Library, 2015, 1740, DOI: 10.1557/op1.2015.1204; Kaur et al., J. Electrochem. Soc., 2015, manuscript accepted for publication). However, its synthesis requires three steps, the third of which is low yielding. Focusing studies on easily-scalable materials, research was therefore targeted to produce products that could be prepared in a single step from commercially available components.
Phenothiazines are generally stable, electron-donating electro-active materials with potential use in energy collection and storage applications and in electrochemically mediated synthesis. To be practical as electron-donating electro-active catholytes for non-aqueous redox flow batteries, solutions of high capacity are required. The present invention described herein provides highly soluble, liquid phenothiazines containing methoxy-terminated ether and oligoether substituents with high diffusion coefficients and robust performance in electrochemical measurements. Further, the catholyte solutions described herein can be synthesized in one step from commercially-available starting materials, thereby circumventing previous synthetic limitations.